The impact of modern headlamps on the design of sag vertical curves

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Abstract

Incorporating safety in the design of a highway is one of the foremost duties of a design
engineer. Design guidelines provide standards that help engineers include safety in the
design of various geometric features. However, design guidelines are not frequently
revised and do not accommodate for the frequent changes in vehicle design. One such
example is the change in vehicle headlamps. These changes significantly impact the
illuminance provided on the road and in turn the design formula.
Roadway visibility is critical for nighttime driving. In the absence of roadway
lighting, vehicle headlamps illuminate the road ahead of a vehicle. Sag vertical curve
design depends on the available headlight sight distance provided by the 1 degree
upward diverging headlamp beam. The sag curve design formulas were developed in
the early 1940s when sealed beam headlamps were predominant. However, headlamps
have changed significantly and modern headlamps project less light above the horizontal
axis. In this research, the difference in illuminance provided by sealed beam headlamps
and modern headlamps was examined. For the theoretical analysis, three different sag
curves were analyzed. On these curves, about 26 percent reduction in illuminance was observed at a distance equal to the stopping sight distance when comparing sealed beam
to modern headlamps. A change in the headlamp divergence angle from 1.0 degree to
0.85 degree will provide the required illuminance on the road when using modern
headlamps. A field study was performed to validate the theoretical calculations. It was
observed that for modern headlamps, a divergence angle less than 1 degree and greater
than 0.5 degrees will provide illuminance values comparable to sealed beam headlamps.
As a part of this research, a preliminary study, examining the impact of degraded
headlamp lenses on the illuminance provided on sag vertical curves was conducted. A
significant reduction in illuminance reaching the roadway on sag curves was observed,
due to headlamp lens degradation.